1. Field of the Invention
The present invention relates to a preparation method of 2,2xe2x80x2-bi-1H-imidazole.
2. Description of the Background Art
2,2xe2x80x2-Bi-1H-imidazole is a compound, which has been widely used as a chelating agent for a metal, an intermediate for a biologically active substance, a component of polyurethane, an intermediate for a copying toner, an intermediate for a high energy substance or the like.
A synthetic method of 2,2xe2x80x2-bi-1H-imidazole currently and widely used is the one proposed by B. F. Fieselmann in 1978. In this method, 20 wt % aqueous glyoxal solution is reacted with anhydrous ammonia gas to obtain 2,2xe2x80x2-bi-1H-imidazole in a brown-colored precipitate with 33% yield (See Inorg. Chem. 1978, 17, 2078). The Fieselmann""s method can be represented by the following reaction scheme 1. 
However, because the above method uses a toxic ammonia gas, it is difficult to control its reaction. The chemical reaction vigorously takes place around an inlet for an ammonia gas injection and then produce precipitate there, which blocks an entrance of the inlet, and accordingly, it is difficult to proceed the reaction. Besides, the ammonia should be used excessively since it is difficult to exactly determine the amount of the ammonia gas input during the synthetic process. Moreover, the yield of the desired product is only 33%.
Therefore, it is certain that the conventional preparation method of 2,2xe2x80x2-bi-1H-imidazole preparation method has disadvantages in aspects of safety and economical efficiency, which are necessary to be improved. Accordingly, a lot of efforts have been made to overcome such disadvantages of the conventional method, for example, by using a catalyst and the like. However, it has not been fundamentally improved yet.
Therefore, an object of the present invention is to solve the problems of the prior art and to provide a preparation method of 2,2xe2x80x2-bi-1H-imidazole through a simple, safe and easily controlled synthetic process with a high yield.
The above and other objects of the invention, as embodied and broadly described herein, can be achieved by using glyoxal and an ammonium salt as starting materials for obtaining 2,2xe2x80x2-bi-1H-imidazole.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.
A preparation method of 2,2xe2x80x2-bi-1H-imidazole of the present invention is carried out through a reaction of glyoxal with an ammonium salt represented by the following reaction scheme 2: 
wherein, NH4+A represents an ammonium salt.
Hereinafter, an embodiment of carrying out the present invention will be described in more detail.
At first, a slurry containing excess amount of an ammonium salt is made by adding the ammonium salt in water. An aqueous glyoxal is then slowly added dropwise to the ammonium salt slurry while vigorously stirring. The resultant is then additionally stirred at room temperature for 0-5 hours. After the reaction is completed, an aqueous alkali solution or distilled water is then added to the reaction mixture so as to adjust its pH to be in the range of 5-7. A produced brown colored solid is then filtered, washed alternately with an organic solvent and distilled water several times and then dried, to obtain 2,2xe2x80x2-bi-1H-imidazole.
In the present invention, glyoxal and an ammonium salt are used as starting materials. 20 wt % aqueous glyoxal is preferably used as a material for the glyoxal. The ammonium salt used as a starting material is not specially limited to any specific compounds. Examples can include ammonium acetate, ammonium benzoate, ammonium bromide, ammonium carbonate, ammonium chloride, ammonium chromate, ammonium dichromate, ammonium formate, ammonium nitrate, ammonium oxalate, ammonium sulfamate, ammonium sulfate, ammonium sulfite, ammonium tartarate, ammonium tetrafluoroborate, ammonium thiocyanate, ammonium thiosulfate and the like.
A temperature when adding the aqueous glyoxal to the ammonium salt slurry is preferably in the range of 30-60xc2x0 C. If the temperature exceeds this range, the yield of the desired product is lowered, and ammonia gas is generated caused by a decomposition of the ammonium salt. The time for adding the aqueous glyoxal to the ammonium slurry depends on a reaction condition and a quantity of the reactants, and is preferably 2-4 hours.
The alkali solution used for adjusting pH of the reaction mixture after completion of the reaction is not specially limited. Examples can include aqueous ammonia, and aqueous solutions of lithium hydroxide, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, sodium acetate and the like. In this case, the concentration of the alkali solution is 5-20% by weight. Distilled water can be used for neutralization instead of an alkali solution. When neutralizing, it is important not to make pH of the reaction mixture exceed 7. If pH exceeds 7, 2,2xe2x80x2-bi-1H-imidazole can be a metal salt so that it can be partially dissolved in water.
According to the present invention as described above, 2,2xe2x80x2-bi-1H-imidazole having a purity of not less than 97% can be obtained in 43-54% yield.